Effects of Treating Variables on Absorption and Distribution of Chemicals in Pine Posts Treated By Double Diffusion

Introducing insoluble toxic materials in wood by double diffusion requires simple equipment, uses inexpensive chemicals, gives clean paintable surfaces, and leaves the wood in a condition not altered by leaching. Results after 11 years of ground exposure have been highly encouraging. In recent treatments, southern pine posts, 7 feet long, peeled and having had 6 inches cut off the butt end were placed butt down in a solution of a first chemical, usually copper sulfate at 5-10 percent concentration for 1 to several days, then in a second solution, usually 3-5 percent of sodium arsenate, phosphate, borate, or fluoride for several days and then seasoned. Several replicates were sectioned for analyses and for distribution of chemical. Although copper sulfate is the most effective first chemical, because of its corrosiveness containers must be made of wood or concrete. The arsenate in the second salt contributes significantly to the toxicity of the precipitated salt. However, sodium fluoride and borate are satisfactory second salts. Uptake of chemical is faster by green wood than by kiln-dried wood, indicating the importance of diffusion over that of capillarity. The rate of uptake, however, decreases with time. The uptake of the second chemical is much more erratic and not too well understood. Temperature was not investigated, but probably would increase the rate of uptake. Slow grown pines took up chemicals more slowly than fast grown pines. Distribution of the precipitated chemical is more uniform with more dilute treating solution. Further improvement in uniformity of treatment is obtained by holding the treated posts under nondrying conditions for a month before seasoning. Total chemical uptake of 0.3 to 1.5 lb./ft.3 are readily obtained. A few tests with hardwoods indicate that the treatment is less effective for hardwoods. This is probably due to greater tolerance to chemicals by those fungi that attack hardwoods.

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